Dimmer circuits are used to control the power provided to a load such as a light or electric motor from an alternating current (AC) power source such as mains. Such circuits often use a technique referred to as phase controlled dimming. This allows power provided to the load to be controlled by varying the amount of time that a switch connecting the load to the power source is conducting during a given cycle.
For example, if voltage provided by the power source can be represented by a sine wave, then maximum power is provided to the load if the switch connecting the load to the power source is on at all times. In this way the, the total energy of the power source is transferred to the load. If the switch is turned off for a portion of each cycle (both positive and negative), then a proportional amount of the sine wave is effectively isolated from the load, thus reducing the average energy provided to the load. For example, if the switch is turned on and off half way through each cycle, then only half of the power will be transferred to the load. Because these types of circuits are often used with resistive loads and not inductive loads, the effect of repeatedly switching on and off power will not be noticeable as the resistive load has an inherent inertia to it. The overall effect will be, for example in the case of a light, a smooth dimming action resulting in the control of the luminosity of the light. This technique will be well understood by the person skilled in the art.
Modern dimming circuits generally operate in one of two ways—leading edge or trailing edge.
In leading edge technology, the dimmer circuit “chops out” or blocks conduction of electricity by the load in the front part of each half cycle (hence the term “leading edge”).
In trailing edge technology, the dimmer circuit “chops out” or blocks conduction of electricity by the load in the back part of each half cycle.
FIG. 1A shows a representation of the function of a leading edge dimmer, while FIG. 1B shows the function of a trailing edge circuit.
In FIG. 1A, the shaded region of the sine wave, representing the applied AC power to the load, indicates the part of the cycle during which the dimmer circuit allows electricity to reach the load. The blank region in front of the shaded region indicates the part of the cycle that has been blocked by the dimmer circuit, preventing power from being applied to the dimmer circuit.
In FIG. 1B, the reverse situation, for the trailing edge, is illustrated. In this case, the shaded region at the beginning of the AC cycle indicates the part of the cycle during which the dimmer circuit allows electricity to reach the load. The blank region after the shaded region indicates the part of the cycle that has been blocked by the dimmer circuit, preventing power from being applied to the dimmer circuit.
Which of the two technologies used for a particular application depends upon the type of the load. Inductive load types (such as iron core low voltage lighting transformers and small fan motors) are best suited to leading edge operating mode, where the established half-cycle load current is terminated when at substantially low levels, thus avoiding undesirable voltage spiking. Capacitive load types are best suited to trailing edge operating mode, where the start-of-half-cycle applied load voltage ramps up from zero at a relatively slow rate, thus avoiding undesirable current spiking.
In practice, it has been necessary to select the appropriate dimmer for the appropriate load. This requires stocking multiples of each type of dimmer and has the risk that the incorrect dimmer is connected to a given load.
An improved form of dimmer circuit, known as “adaptive” or “universal” dimmers has been developed, which can function in either the leading edge or trailing edge mode. This alleviates the need to have multiples of each dimmer type to cater for different loads, and the installer does not have to be particularly concerned about the load type. Additionally, from the manufacturing standpoint, only one build type of dimmer is required.
Universal dimmer designs incorporate a means to initially determine which operating mode is suitable for the connected load, in addition to non-volatile memory elements for the purpose of retaining the operating mode thereafter.
In many existing devices, the load detection process and resultant mode changeover (if necessary), often results in lamp load flickering which can be undesirable in lighting installations.
It is an object of the present invention to provide a circuit that is able to detect the presence of an inductive load.